The invention is concerned with a method and an apparatus for pressing plasticized plastic material into a shaping cavity in order to produce tubes to be used as containers for fluent products.
For producing molded articles consisting of plastic, such as covers or caps for containers, and particularly the production of head pieces for packing tubes, a pressing procedure is known as an alternative to the injection molding procedure. In the pressing procedure, in contrast with the injection procedure, plasticized plastic material is introduced into a shaping cavity to be formed toward and against it, using a limited amount of pressure. Since pressing procedures quite generally can operate with lower temperatures of the plasticized plastic material than injection procedures can use, they have the advantage, in addition to the fact that they use smaller quantities of energy, of having shorter cooling-off times, and as a result the mold can be opened sooner and the cavity-emptying step in the manufacturing can be shortened by that means.
In the familiar pressing procedure, the mold is made out of a die and a shaft, with the shaft always bearing a tube-shaped tube element made of plastic. One end of this tube-shaped tube element penetrates into the die, together with the shaft, and seals off the shaping cavity at the circumference of the shaft when the mold is closed. The plasticized plastic formed into the shape of a head piece for a tube joins with the edge of the tube-shaped tube element which is penetrating into the mold under the influence of its heat of fusion.
In the familiar procedure, the plasticized plastic is introduced into the die in the shaping cavity, which is still open, through a central opening, by means of a material supply head from the side opposite the shaft, or discharged from the material supply head in the form of a ring. The plasticized material adhering to the material supply head in the form of a ring is stripped off when it is withdrawn from the shaping cavity through the die, and then it continues to adhere to a surface adjoining the shaping cavity until the pressing step takes place.
Defects occur in the container tube products produced in accordance with this known procedure in certain cases, and some of those are perceptible on the surface. An examination of the processes involved in pressing has shown that plastic material which comes into direct contact with the cooled die while being stripped off of the material supply head and is formed by the matrix, with the plastic being next to the die over a relatively large surface, can assume a limited crystalline condition in places. As a result of the further cooling which takes place during the pressing process, the remaining heat of fusion obviously no longer sufficies, in all cases, to plasticize material again which is already crystallized.
The invention is based on the realization that, when the plasticized material is stripped off of the material supply head, a contact with the cooled matrix involving a comparatively large surface can come about, and that contact results in a comparatively great cooling off in that area of contact until the mold is closed and the plastic is pressed.